1. Field of the Invention
The invention relates a method for operating a density measuring device having a measuring tube for determining the density of a medium, with the measuring tube being filled with the medium, the measuring tube being incited to vibrate, the frequency of the vibration being determined and the density of the medium being calculated from the determined frequency and other parameters which determine physical-geometrical properties of the measuring tube. The invention additionally relates also to a device for density measurement on the basis of vibration analysis, having at least one measuring tube, a medium which is situated in the measuring tube, at least one vibration generator which acts on the measuring tube, at least one measured value sensor for measuring the vibration of the measuring tube, and having an evaluating unit for evaluating the vibration measured by the measured value sensor and calculating a density value of the medium from the measured vibration and other parameters which determine physical-geometrical properties of the measuring tube.
2. Description of Related Art
Methods for measuring the density of a medium which is situated in a hollow body—such as, for example, in a measuring tube, in which the hollow body, whose vibration behavior is substantially known, is incited to vibrate, and the resulting vibration, in particular, the frequency of the resulting vibration, is incorporated for the determination of the density of the medium situated in the hollow body, are long known and are widely used.
Where it is referred to that the present invention relates to a method for operating a density measuring device, with the method being based on the principle of vibration analysis, this means all devices which can also be used in any case for density measurement, even if not provided primarily for density measurement. For example, it is known to also carry out a density measurement with Coriolis mass throughflow meters, with such mass throughflow measuring devices also having a measuring tube which is filled with a medium or is traversed by said medium and which is incited to vibrate, but wherein, for the determination of the mass throughput, it is not the frequency of a natural vibration which is evaluated, but primarily the phase difference between two sections of the measuring tube which is vibrating. Mass throughflow measuring devices which are based on the Coriolis principle are nonetheless fundamentally suitable for being operated in the manner of a density measuring device.
The density determination is based fundamentally on the knowledge that a mechanical system which is capable of vibration can be mathematically described in the simplest case by a simple spring-mass system, with the natural frequency of the system being in a certain functional relationship with the mass and other physical-geometrical properties of the system, such as, for example, the spring constant by means of which the relevant behavior of the spring can be mathematically depicted in the simplest case. In an idealized, purely linear spring-mass system, the natural angular frequency corresponds to the square root of the quotient of the spring constants of the spring and the overall mass of the system. If the overall mass of the system is composed, as in the case considered here, of the mass of the measuring tube and the mass of the medium situated in the measuring tube, then the contribution of the medium contained in the measuring tube to the overall mass is equal to the product of the density of the medium and the volume which is enclosed by the measuring tube and which is filled by the medium. It is directly evident that, after the determination of the frequency of the natural vibration of the hollow body—in this case of the measuring tube—by means of measurement, it is possible to determine the density of the medium enclosed by the measuring tube, assuming that the volume of the measuring tube is known.
On closer inspection, however, it has been proven in practice that the above-described modeled description of a density measuring device having a measuring tube for determining the density of a medium, with the measuring tube being filled with the medium and the measuring tube being incited to vibrate, is only a first and if appropriate insufficient modeled description of the actual conditions. It has, for example, been proven that—even for incompressible media—the density of the medium obtained from the simple approach by means of a spring-mass system also exhibits a dependency on the pressure prevailing in the measuring tube, and therefore, in the medium. It is also known that the density measurement is temperature-dependent, and specifically, to a greater extent than can be explained by a change in volume of the medium under the influence of temperature.